The present invention relates to a speaker unit, and more particularly, to a speaker unit having a voice coil wound to a length greater than the length of the magnetic gap in which the coil is disposed.
A conventional speaker of the type contemplated by the present invention is illustrated in FIG. 1. The unit shown in FIG. 1 has a pole yoke 1 with a inverted-T cross section and which has around its bottom an annular magnet 2 that is magnetized in the direction of its thickness. A plate 3 rests on the magnet 2 to form a magnetic gap with the peripheral wall of the center pole 1A of the pole yoke 1. A voice coil 5 wound around a bobbin 4 is inserted into the magnetic gap. The bobbin 4 is supported on a frame 7 by a damper 6 that permits vibratory movement of the bobbin. A diaphragm 8 with a center cap 9 is connected to the bobbin 4. The outer periphery of the diaphragm 8 is supported by an edge portion 11 that is fixed to the frame 7 by a gasket 10. In order to make effective use of the magnetic flux in the gap, the voice coil 5 is wound around the bobbin 4 to a length (in the longitudinal direction of the coil) greater than the magnetic gap length. The center pole 1A of the pole yoke 1 is fitted with a metal cap 21 provided to reduce the distortion that is introduced into the coil current by the inductance of the voice coil 5.
The operation of the speaker unit in FIG. 1 is as follows. When a current flows through the voice coil 5, either an upward or downward driving force acts on the coil 5 depending upon the direction of the current flow. Since the coil 5 is mobile, the driving force acting on the coil 5 is transmitted to the diaphragm 8 through the bobbin 4. As a result, the diaphragm 8 pushes the air in front of it, creating sound waves.
The driving force F acting on the coil 5 is expressed as: EQU F=Bli,
where B is the density of magnetic flux linking the coil 5, l is the effective length of the coil 5, and i is the current through the coil 5.
FIG. 2A shows the coil 5 positioned so that its center in the widthwise direction coincides with the center of the magnetic gap, and FIG. 2B shows the corresponding profile of magnetic flux density linking the coil 5. FIG. 3A shows the coil 5 positioned in such a manner that its center in the widthwise direction is off the center of the magnetic gap, and FIG. 3B shows the corresponding profile of magnetic flux density linking the coil 5. The area of the hatched portion in each of FIGS. 2B and 3B represents the driving force F acting on the coil 5. As can readily be seen, the hatched area in FIG. 3B is smaller than that in FIG. 2B. In other words, the value of the force factor Bl as a function of the displacement of the coil 5 is not constant within the limits of the oscillatory movement of the coil, but rather, as shown in FIG. 4, the value of Bl, and hence the driving force F, decreases as the center of the coil 5 in the lengthwise direction moves away from the center of the magnetic gap.
Since the force factor Bl as a function of the displacement of the coil 5 is not constant within the limits of the oscillatory movement of the coil, the conventional speaker unit does hot produce a sinusoidal driving force F, even if the input is a sine wave. Instead, a complex wave containing second and third harmonics is produced, and the resulting third-harmonic distortion is one of the major sources of distortion in the sound radiated from the speaker in the range of low sound level.